Method and apparatus for triggering the inspiratory phase of a respirator

ABSTRACT

A method and apparatus are herein disclosed for triggering the inspiratory phase of a respirator in response to an inhalation attempt on the part of a patient being &#39;&#39;&#39;&#39;breathed.&#39;&#39;&#39;&#39; The invention is used on respirators where a positive end expiratory pressure is provided the patient at the end of his exhalation, that is, the patient is prevented from exhaling completely by the respirator retaining a net positive pressure within the patient&#39;&#39;s lungs. To sense an inhalation attempt in such respirators and to simultaneously distinguish such an attempt from a leak in the patient&#39;&#39;s breathing line, a patient trigger system is provided which generates an output signal in response to differential pneumatic inputs including a variable reference pressure input and a breathing line pressure input. The reference pressure input comprises a delayed breathing line pressure which is communicated to the triggering device during a portion of a patient&#39;&#39;s exhalation phase so that the reference pressure closely approaches and tracks the breathing line pressure toward the end of the exhalation phase. At this point, an inhalation attempt even on the part of a feeble patient is operable to trigger the system which, in turn, shifts the respirator into an inspiratory assistor mode. The net operative result of such a triggering system is the generation of an appropriate output signal only in response to breathing line pressure decay &#39;&#39;&#39;&#39;rates&#39;&#39;&#39;&#39; associated with a patient&#39;&#39;s attempt to inhale.

Unite States Patent [1 1 Cibulka July 29, 1975 METHOD AND APPARATUS FOR TRIGGERING THE INSPIRATORY PHASE OF A RESPIRATOR [75] Inventor: Anthony B. Cibulka, Poynette, Wis.

[73] Assignee: Airco, Inc., Montvale, NJ.

[22] Filed: July 27, 1973 [21] Appl. No.: 383,422

[52] US. Cl 128/145.8; 128/188 [51] Int. Cl. A61M 16/00 [58] Field of Search l28/145.8, 142, 142.3,

Primary ExaminerRichard A. Gaudet Assistant Examiner-Henry J. Recla Attorney, Agent, or Firm-Roger M. Rathbun; Edmund W. Bopp; H. Hume Mathews [57] ABSTRACT A method and apparatus are herein disclosed for triggering the inspiratory phase of a respirator in response to an inhalation attempt on the part of a patient being breathed. The invention is used on respirators where a positive end expiratory pressure is provided the patient at the end of his exhalation, that is, the patient is prevented from exhaling completely by the respirator retaining a net positive pressure within the patients lungs. To sense an inhalation attempt in such respirators and to simultaneously distinguish such an attempt from a leak in the patients breathing line, a patient trigger system is provided which generates an output signal in response to differential pneumatic inputs including a variable reference pressure input and a breathing line pressure input. The reference pressure input comprises a delayed breathing line pressure which is communicated to the triggering device during a portion of a patients exhalation phase so that the reference pressure closely approaches and tracks the breathing line pressure toward the end of the exhalation phase. At this point, an inhalation attempt even on the part of a feeble patient is operable to trigger the system which, in turn, shifts the respirator into an inspiratory assistor mode. The net operative result of such a triggering system is the generation of an appropriate output signal only in response to breathing line pressure decay rates associated with a patients attempt to inhale.

13 Claims, 13 Drawing Figures PATENTED JUL29 H175 SHEET PHASE I NSPIRA TORY TIME (PRIOR ART) Fl 6 2 $5 wake $3.8m

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PATENTEBJULZQIHTS 895,800 saw 3 FlCzulO METHOD AND APPARATUS FOR TRIGGERING THE INSPIRATORY PHASE OF A RESPIRATOR BACKGROUND OF THE INVENTION The present invention relates to respirators and, morespecifically, to a method and apparatus for triggering the inspiratory phase of a respirator of the type which retains an exhalation positive plateau pressure in the patients lungs at the end of expiration only in response to a patients attempt to inhale.

The intensive care units of most medical institutions, today, include equipment for assisting or sustaining the critical functions of patients who are too feeble, sick or injured to sustain their own functioning. One such critical function, of course, is breathing.

For some time now, respirators have been used to assist or even to entirely control, through forced breathing, a patients breathing cycle. In particular, a gas including oxygen or pure oxygen may be communicated with a patients lungs, under pressure, in response to that patients attempt to inhale. Typically, the patients breathing line is provided with a pressure sensor which actuates a means for communicating pressurized gas to the patients lungs in response to the sensing of a predetermined increment of negative pressure in the breathing line.

For most patients, a respirator having a pressure sensor which responds to a negative or slight vacuum pressure in the patient breathing line is satisfactory, however, in some patients it is necessary to retain a positive pressure within the patients lungs for reasons such as preventing alveolar collapse. In such instances the respirator should be triggered by a pressure drop in the patient line when the patient attempts to inhale, however, the pressure within the patient line may not drop below atmospheric pressure but should trigger at some positivepressure. The basic triggering sensors, heretofore used, are thus ineffective to sense a pressure drop occurring wholly above atmospheric pressure, particularly in patients that are too feeble to overcome the positive plateau pressure in order to draw a vacuum to actuate the sensor and shift the respirator into an inspiratory phase.

Prior efforts in overcoming these problems have included various means for biasing the trigger so that a patient can shift the respirator into an inspiratory phase without having to draw a vacuum but merely by making some attempt at inhalation.

However, one drawback of such biased trigger systems has been that the biasing force must be pre-set and bears no relationship to a varying patient breathing line pressure which variation may be caused by a slow or moderate leak. Accordingly, an inherent disadvantage accompanies such systems, in that a slow or moderate leak, which would not be detected by a low pressure alarm system, may produce a pressure drop increment which would be sensed by the triggering system as a patients effort to inhale. This, in turn, would cause the premature shifting of the respirator into an inspiratory phase.

Therefore, it would be advantageous if a method and apparatus were provided for discriminately shifting a respirator into an inspiratory phase only in response to a patients effort to inhale and independent of any slow or moderate leaks which may occur in the patients breathing line.

SUMMARY OF THE INVENTION The foregoing drawbacks in present respirator triggering methods and apparatus are overcome by providing a respirator which is capable of maintaining a positive end expiratory pressure in the patients lungs at the end of exhalation and yet which can safely and effectively be triggered by an attempt by the patient to inhale to shift the respirator into the inhalation mode, where the pressure in the patient line need not go below atmospheric pressure but may be retained at a positive pressure, yet a gradual depletion of pressure in the patient line, such as may be caused by a leak in that line, will not cause triggering of the respirator.

In the present method and apparatus, triggering is achieved in a positive end expiratory pressure respirator by the provision of a patient triggering system having differential inputs for detecting the rate of pressure decay. A first input represents the patients breathing line pressure while a second input represents a variable reference pressure consisting of a delayed breathing line pressure obtained during most of the patients exhalation phase. The delayed breathing line pressure gradually approaches the breathing line pressure even when a leak exists in the breathing line. Accordingly, the system automatically adjusts for breathing line pressure decay and will actuate the inspiratory cycle only in response to a predetermined rate of drop in breathing line pressure with respect to the variable reference pressure.

BRIEF DESCRIPTION OF THE DRAWINGS While the invention is particularly pointed out and distinctly claimed in a concluding portion of the specification, a preferred embodiment is set forth in the following detailed description which may be best understood when read in connection with the accompanying drawings in which:

FIG. 1 is a graph illustrating a patients breathing cycle under a controller mode of a respirator wherein the inspiratory phase is initiated independently of the patients efforts to inhale;

FIG. 2 is a graph illustrating a patients breathing cycle under an assistor mode wherein an inspiratory phase is initiated by a patients efforts to inhale;

FIG. 3 is a diagrammatic illustration of a prior art, non-differential patient trigger and respiratory system which may be utilized to produce the patients breathing cycle illustrated in FIG. 2;

FIG. 4 is a graph illustrating a patients breathing cycle when assisted by a respirator providing positive pressure to the patients lungs at the end of an expiratory phase and triggered by the patients efforts to inhale;

FIG. 5 is a diagrammatic illustration of a prior art, mechanically biased, non-differential patient trigger utilized to produce the patient breathing cycle set forth in FIG. 4;

FIG. 6 is a diagrammatic illustration of a prior art, pneumatic, differential patient trigger device having means for pre-setting a reference pressure input;

FIG. 7 is a graph illustrating a patients breathing pressure cycle utilizing the triggering devices shown in FIGS. 5 and 6 when a small leak exists in the patients breathing line;

FIG. 8 is a graph of a patients breathing cycle utilizing the patient trigger devices shown In FIGS. 5 and 6 3 when a relatively large leak exists in the patients breathing line;

' FIG. 9 is a diagrammatic illustration of apparatus according to the present invention as arranged during an inspiratory portion of an operating cycle;

FIG. 10 is a diagrammatic illustration of the switching valve portion of the apparatus of FIG. 9 as arranged during an expiratory portion of an operating cycle;

FIG. 11 is a graph illustrating a patients breathing cycle during the operation of the triggering device of the present invention illustrated in FIGS. 9 and 10;

FIG. 12 is a graph illustrating a patients breathing cycle utilizing the apparatus according to the present invention with a breathing line having a small leak; and

FIG. 13 is a graph of a patients breathing cycle utilizing the apparatus and method according to the present invention in association with a breathing line having a relatively large leak.

THE PRIOR ART Referring now to the drawings which like numerals are used to indicate like parts throughout the various views thereof, FIG. 1 presents a graph illustrating a patients breathing cycle while a respirator is in a controller mode and the patients inspiratory phase is initiated independently of any effort to inhale.

As can be seen, the controller mode induces forced breathing to a patient and is especially helpful when a patient is virtually unable to initiate or sustain breathing on his own. Of course, the breathing cycle induced under the controller mode is not necessarily synchronized with the natural breathing cycle of the patient and may make the patient hypoxic or may cause hyperventilation. Therefore, it is desirable to have the patients breathing cycle controlled by the patients natural demand for air and in accordance with the patient's normal inspiratory and expiratory characteristics.

For this reason, most respirators provide the abovementioned controller mode only during such times as the patient is incapable of generating minimum breathing activity. Accordingly, a patient actuated trigger mechanism is generally provided for triggering an inspiratory phase upon the sensing of an attempt on the part of the patient to inhale.

FIG. 2 shows a patients breathing cycle while under a respirator operating in an assistor mode. Generally, the patient trigger is preset so that the sensing of a certain incremental negative pressure s is operable to actuate the trigger, which, in turn, shifts the respirator into an inspiratory phase.

It will be observed, that the cycle shown in FIG. 2 requires that the patient draw a negative pressure, i.e., a

vacuum, before the respirator initiates an inspiratory phase. Had the patient not been able to draw the vacuum required, the increment s would not have been produced and the machine would have operated in the controller mode illustrated by FIG. 1 by means of apparatus well known in the art. In this sense, the controller mode overrides the assistor mode when the patient is unable to generate the necessary increments to initiate the inspiratory phase of his breathing cycle.

FIG. 3 sets forth a diagrammatic representation of prior art apparatus which may be utilized to achieve the patient breathing pressure curve set forth in FIG. 2. Es=

sentially,'the apparatus includes a housing 10 defining a breathing line pressure input chamber 12 which is en= closed by a flexible diaphragm member 14. A gas line l6 communicates the chamber 12 with the patients breathing line 18 which, in turn, communicates with the respiratory unit 20. As used herein, the term patients breathing line includes any gas line in communication with the patients lungs for delivery of gas thereto, and may also include any further gas line adapted to track the pressure in the patients lungs.

A respirator control unit 22 may be provided and is actuated in response to the movement of the flexible diaphragm 14. Any one of various different devices may be used to detect the movement of the diaphragm 14 in response to the patients drawing of a vacuum in the chamber 12. Such movement detectors include sensitive electrical switches, direct electrical contact of points (one of which moves with the diaphragm), light source and photo-cell arrangements, and fluidic detectors.

For the purpose of diagrammatic illustration only, a member 24 is shown to extend from the flexible diaphragm 14 into electrical switching relationship with the electrical controller unit 22.

While the above described triggering scheme is satisfactory for most applications, it is sometimes desirable to keep a positive pressure in the patient's lung alveoli at all times. In such cases, a positive pressure is maintained in the patient's lungs at all times, including the entire expiratory phase.

FIG. 4 is a graph of a patient's breathing cycle wherein positive pressure is maintained in the lungs at all times. Pressure curve 26 represents the minimum positive pressure to be maintained within the patient's lungs except for a momentary decrease in pressure occasioned by the patients effort to inhale. The pressure level 26 is generally referred to as the positive end expiration pressure and hereinafter will be referred to as the PEEP.

It can be seen by reference to FIG. 4, that when a PEEP is maintained in a breathed patient, an appara= tus such as that illustrated in FIG. 3 is ineffective in that it would be difficult for even a healthy patient to draw sufficient vacuum within the chamber 12 to initiate the inspiratory phase of his breathing cycle. Therefore, a prior art apparatus such as that illustrated diagrammat= ically in FIG. 5 is sometimes utilized to assist the pa= tient in initiating an inspiratory phase.

More specifically, FIG. 5 illustrates a modification of the apparatus shown in FIG. 3 wherein a mechanical bias is applied to the flexible member 14. The bias is of the same polarity as the force created by a patient's at= tempt to inhale so as to assist the patient in movingthe flexible diaphragm against the positive pressure within chamber 12. Accordingly, a spring 28, or the like, may be operatively associated with the diaphragm 14 to urge the diaphragm toward an inspiratory phase initiat= ing position. The magnitude of the bias force applied by the spring 28 may be adjusted by a lead screw 30, or the like, so as to pre-set the pressure drop increment neces= sary to move the diaphragm 14.

The apparatus of FIG. 6 operates essentially in the same manner as the apparatus of FIG. 5. A pneumatic bias is applied to the diaphragm 14 by supplying pres= surized gas to a reference chamber 32 formed in the overall housing 10 of the trigger assembly which refer= ence pressure may be adjusted by means of a regulator valve 34. By applying a positive reference pressure to the chamber 32, the flexible diaphragm 14 may be moved in the actuating direction in response to a pres= sure drop in chamber 12 which causes the pressure therein to be reduced a predetermined pressure increment below the reference pressure.

While the operation of the triggers illustrated diagrammatically in FIGS. 5 and 6 is generally satisfactory when a positive pressure is maintained within a patients lungs under ideal conditions, if a leak should develop in the patients breathing line, such prior art triggers are likely to initiate an inspiratory phase prematurely.

In particular, by reference to FIG. 7, it can be seen that when a small leak develops in the patients breathing line, the pressure in the patients breathing line may fall the s increment beneath the reference pressure (which in this case is PEEP) and initiate an inspiratory phase prematurely. FIG. 8 illustrates the condition that would exist if a larger leak were to develop within the patients breathing line which leak may not be sufficiently large to actuate a low pressure alarm or other alarm system, typically installed in existing prior art respirators.

It can further be seen by reference to FIG. 8, that a larger leak would initiate the inspiratory phase entirely too early putting the respirator machine into a free running'condition which is liable to cause hyperventilation in the patient.

Clearly, such a condition is highly undesirable.

THE PREFERRED EMBODIMENT OF THE PRESENT INVENTION Referring now to FIGS. 9 and 10, apparatus according to the present invention is shown diagrammatically to include a housing 10 having a flexible diaphragm 14 mounted therein to define a breathing line pressure chamber 12 and a variable reference pressure chamber 32. A gas line 16 communicates the chamber 12 with the patients breathing line 18 which, in turn, communicates with the respiratory unit 20.

The reference pressure chamber 32 is connected with the patients breathing line 18 via lines 16, 17 and 36. A pneumatic resistor 38 and a pneumatic capacitor 40 are operatively connected within the lines 17 and 36, respectively, to delay the pressure build-up in the reference pressure chamber 32 when the chamber 32 is communicated with the pressure of breathing line 18. A switching valve 42 is operable to vent the reference pressure chamber 32 to atmosphere through tube 46 when in the position illustrated in FIG. 9 and to cornmunicate the reference pressure chamber 32 to the patients breathing line 18, when in the position illustrated in FIG. 10. When in this last mentioned position, both the reference pressure chamber 32 and the compliant pneumatic capacitor 40 are gradually pressurized to the pressure existing in the line 18.

The system of the present invention functions by varying the reference pressure in response to variations in the pressure of the breathing line 18 so that the trigger will actuate in response to a pressure drop at rates which are consistent with the rates produced by efforts of patients to inhale but will not actuate an inspiratory phase in response to pressure drop at rates consistent with small or moderate leaks in the overall system supplying the patient.

For illustration purposes, FIG. 11 shows a pressure curve (solid line) for a patients breathing cycle while the desired PEEP comprises a broken line curve 44. The reference pressure curve showing the pressure in chamber 32 is indicated as dotted line 46. It should be kept in mind that an inspiratory phase will be initiated when the solid pressure curve falls s below the reference pressure curve 46.

During the inspiratory phase, the valve 42 is switched to vent the reference pressure chamber to atmosphere through the exhaust port 46.

A solenoid 48 may be connected to the switching valve 42 so that the valve 42 is maintained in the venting position shown in FIG. 9 during a patients inspiratory phase. The solenoid may be activated at some time after the start of .the expiratory phase. However, the switching of the valve 42 to the position shown in FIG. 10 must be delayed for a time period T This period should generally be in the range of 0.2 to 0.5 seconds.

During the time period T the pressure in a patients lung and airway decreases to approximately the ideal PEEP level 44. At the end of period T the switching valve 42 is switched to the position as shown in FIG. 10, thereby connecting the trigger reference chamber 32 and the pneumatic capacitor 40 to the patients airway and lungs by way of the patient breathing line 18 and the pneumaticresistor 38. The resulting exponential increase in pressure in the pneumatic capacitor 40 and trigger reference chamber 32 is gradual because of the time delaying effect of the capacitor 40 and the pneumatic resistor 38.-.The capacitor 40 may be formed with an elastic compliant wall 50 so as to provide the damping effect of a large capacitor without requiring the large physical dimensions of a rigid wall capacitor having an equivalent damping or time delaying effect.

As can be seen by reference to FIG. 11, the reference pressure curve 46' gradually approaches the pressure level in the patients lungs (solid curve) during the expiratory phase. When the patient attempts to inhale, the pressure in the input pressure chamber 12 drops at a fairly rapid rate. The pressure in the reference chamber 32 cannotdecrease as rapidly due to the time delaying action of the pneumatic resistor 38 and thus the pressure in chamber 12 soon decreases to a pressure s below the pressure in the reference chamber 32 and thereby lifts the diaphragm 14 upwardly to trigger the main respiratory unit 20 to the inspiratory phase.

Referring briefly to FIG. 12, a patients breathing cycle is illustrated during the condition where the patients breathing line has developed a small leak. It can be seen that the reference pressure 46 tends to track the decaying pressure in the breathing line 18 so as to prevent premature initiation of the inspiratory cycle in response to such a leak.

FIG. 13 shows the condition of FIG. 12 wherein a larger leak has developed in the breathing line 18 which leak may be insufficient to actuate an alarm system but may be sufficiently large to eliminate the positive pressure within the patients lungs at the end of the expiratory phase. Even under such an extreme condition, however, it will be seen that the reference pressure nearly tracks the decaying pressure in the patients breathing line and an inspiratory phase is not initiated until the patient attempts to inhale when the breathing tube pressure line (solid line) falls s with respect to the variable reference pressure 46.

In order to insure such operation, the pneumatic resistor 38 should be selected so that the variable reference pressure curve 46 has the correct response time.

It can thus be seen that through the synergistic cooperation of the elements comprising the present invention, the apparatus of the preferred embodiment is uniquely uncomplicated so as to be relatively inexpensive in manufacture and safe in operation.

The present invention represents a technical advance in the field in that, through the operation thereof, a respirator operating with PEEP may be discriminately controlled so that the inspiratory phase of a patients breathing cycle will be initiated only in response to the patients attempt to inhale. Therefore, the system is not adversely affected by breathing tube leaks.

While the preferred embodiment is set forth in the foregoing paragraphs, it is of course to be understood that various modifications and changes may be made therein without departing from the invention.

For example, the pressure inputs may be converted into other parameters, such as electrical voltages, which may then be phase oriented as indicated in FIGS. 11-13 and directed as inputs to an electrical triggering device in a manner well known in the electrical arts. Similarly, it is not necessary that the variable reference pressure precisely follow the existing breathing line pressure. All that is required is that the reference pressure substantially track the existing pressure so that some lateral spacing of the two pressure lines is permissible.

Accordingly, it is intended to cover in the following claims all such modifications and changes as may fall within the true spirit and scope of the present invention.

I claim:

1. A method for triggering a respirator from an expiratory phase into an inspiratory phase to force gas to a patient through a breathing line comprising the steps of:

monitoring the pressure within the patient breathing line during the expiratory phase;

sensing the pressure within the patient breathing line during a period which commences a predetermined time period after commencement of the expiratory phase, and

triggering the respirator into the inspiratory phase upon said patients attempting to breathe while preventing triggering due to pressure declines generated by leaks from said system, by effecting said triggering when a drop in said sensed pressure over a sub-interval of said period exceeds a minimum predetermined value; and disabling said triggering irrespective of the level of said sensed pressure where the average rate of pressure decline over said subinterval is less than a second predetermined value.

2. A method according to claim 1 wherein said step of sensing the rate of pressure drop is carried out during a period commencing 0.2 0.5 seconds after commencement of the expiratory phase and ending at the time the respirator is actuated into the inspiratory phase.

3. A method'for triggering a respirator from an expiratory phase into the inspiratory phase to force gas to a patient through a patient breathing line comprising the steps of:

monitoring the pressure within the patient breathing line to provide a first monitored pressure, modifying the first monitored pressure with respect to time to provide a modified pressure whereby the modified pressure substantially tracks the first monitored pressure at low rates of change in the first monitored pressure, and

sensing a predetermined minimum pressure differential between said first monitored pressure and said modified pressure to actuate the respirator into an inspiratory phase.

4. A method according to claim 3 including the step of providing a pressure differential responsive means operatively associated with the respirator and wherein:

said step of monitoring the first pressure within the patient breathing line includes the step of communicating the patient breathing line pressure with the pressure differential responsive means as a first input thereto,

said step of modifying the first monitored pressure to provide a modified pressure includes the step of communicating the patient breathing line pressure with the pressure differential responsive means through a restricted passageway, as a second input thereto during a portion of the expiratory phase, and

said step of sensing a predetermined minimum pressure differential to actuate the respirator into the inspiratory phase comprises the step of triggering the respirator by the operation of the pressure differential responsive means when the predetermined minimum pressure differential is sensed between the first input and the second input.

5. A pressure differential triggering system for shifting a respirator from an expiratory phase into an inspiratory phase in response to a patients effort to inhale, said system comprising:

a patient breathing line providing communication between the patient and the respirator,

first and second enclosed chambers, a flexible diaphragm positioned between said first and second chambers and adapted to be flexed in response to a minimum predetermined pressure differential between the pressure in said first chamber and the pressure in said second chamber,

means for communicating the pressure in said patient breathing line to said first chamber,

modifying means adapted to sense the pressure in said patient breathing line and communicate as a reference pressure to said second chamber, a modified pressure which substantially tracks the patient breathing line pressure communicated to said first chamber,

whereby said modified pressure in said second chamber lags the patient breathing line pressure in said first chamber with respect to time and rapid changes in pressure in said patient breathing line cause said minimum predetermined pressure differential between said first and second chambers to flex said diaphragm, and

means to sense the flexing of said diaphragm to provide an inspiratory phase triggering signal to the respirator.

6. The improvement according to claim 5 wherein said modifying means communicates the modified pressure to the second chamber only during a portion of the expiratory phase and comprises a switching valve:

said switching valve being operated by a valve actuating means, and

said valve actuating means being operable to place said switching valve into a position communicating the modified pressure with the second chamber during a period commencing 0.2 0.5 seconds after commencement of the expiratory phase and terminating upon the commencement of the next inspiratory phase.

7. The improvement according to claim 6 wherein said modifying means includes a pneumatic resistor restricting communicating the pressure sensed in the breathing line with the second chamber.

8. In a respirator for supplying gas to a patient and having inspiratory and expiratory phases, the respirator being adapted to retain a predetermined positive gas pressure within the patient at the end of the expiratory phase and having switching means to switch the respirator from the expiratory phase to the inspiratory phase, the improvement comprising:

a patient breathing line for supplying gas from the respirator to the patient;

means to monitor the pressure in the patient breathing line during the expiratory phase,

means to sense the pressure in said patient breathing line during a period which commences a predetermined time period after commencement of the expiratory phase; and

means to trigger the respirator into the inspiratory phase upon said patients attempting to breathe, said means preventing triggering due to pressure declines generated by leaks from said system by effecting said triggering where a drop in said'sensed pressure over a sub-interval of said period exceeds a minimum predetermined value; and said means further, disabling said triggering irrespective of the level of said sensed pressure where the average rate of pressure decline over said sub-interval is less than a second predetermined value.

9. Apparatus for triggering a respirator from an expiratory phase into an inspiratory phase of a breathing cycle in reponse to an effort on the part of a patient to inhale comprising, in combination:

a patient breathing line communicating between the respirator and the .patient,

pressure differential responsive means,

means for communicating the pressure in said breathing line to said pressure differential responsive means as a first input thereto,

means for modifying the pressure in said patient breathing line and communicating the modified pressure to said pressure differential responsive means only during a portion of an expiratory phase,

as a second input to said pressure differential responsive means,

said modifying means retarding any pressure variations of the second input with respect to time, whereby the modified pressure comprising the second input lags out of phase with respect to the pressure in the breathing line comprising the first input, and

means responsive to a predetermined minimum pressure differential between said first input and said second input in said pressure differential responsive means to trigger the respirator from the expiratory phase into the inspiratory phase.

10. Apparatus according to claim 9 wherein said pressure differential means comprises a housing, a flexible diaphragm separating said housing into a first chamber for receiving said first input and a second chamber for receiving said second input, and wherein said flexible diaphragm flexes in response to the predetermined rate of pressure drop in said first input to trigger the respirator into an inspiratory phase.

11. Apparatus according to claim 10 wherein said means for modifying the pressure in said patient breathing line and communicating the modified pressure to said pressure differential responsive means operates only during a portion of an expiratory phase and comprises a switching valve:

said switching valve being operated by a valve actuating means, and

said valve actuating means being operable to switch said switching valve into a position communicating the second input with said pressure differential responsive means during a period commencing 0.2 0.5 seconds after commencement of the expiratory phase and terminating upon the commencement of the next inspiratory phase.

12. Apparatus according to claim 10 wherein said modifying means includes a pneumatic resistor restricting a gas passageway formed by said means for providing the second input to said pressure differential responsive means.

13. Apparatus according to claim 11 wherein said modifying means includes a pneumatic capacitor comprising a variable chamber in communication with said second chamber, said variable chamber adapted to be reduced in volume in response to a decrease in said second pressure input. 

1. A method for triggering a respirator from an expiratory phase into an inspiratory phase to force gas to a patient through a breathing line comprising the steps of: monitoring the pressure within the patient breathing line during the expiratory phase; sensing the pressure within the patient breathing line during a period which commences a predetermined time period after commencement of the expiratory phase, and triggering the respirator into the inspiratory phase upon said patient''s attempting to breathe while preventing triggering due to pressure declines generated by leaks from said system, by effecting said triggering when a drop in said sensed pressure over a sub-interval of said period exceeds a minimum predetermined value; and disabling said triggering irrespective of the level of said sensed pressure where the average rate of pressure decline over said subinterval is less than a second pre-determined value.
 2. A method according to claim 1 wherein said step of sensing the rate of pressure drop is carried out during a period commencing 0.2 - 0.5 seconds after commencement of the expiratory phase and ending at the time the respirator is actuated into the inspiratory phase.
 3. A method for triggering a respirator from an expiratory phase into the inspiratory phase to force gas to a patient through a patient breathing line comprising the steps of: monitoring the pressure within the patient breathing line to provide a first monitored pressure, modifying the first monitored pressure with respect to time to provide a modified pressure whereby the modified pressure substantially tracks the first monitored pressure at low rates of change in the first monitored pressure, and sensing a predetermined minimum pressure differential between said first monitored pressure and said modified pressure to actuate the respirator into an inspiratory phase.
 4. A method according to claim 3 including the step of providing a pressure differential responsive means operatively associated with the respirator and wherein: said step of monitoring the first pressure within the patient breathing line includes the step of communicating the patient breathing line pressure with the pressure differential responsive means as a first input thereto, said step of modifying the first monitored pressure to provide a modified pressure includes the step of communicating the patient breathing line pressure with the pressure differential responsive means through a restricted passageway, as a second input thereto during a portion of the expiratory phase, and said step of sensing a predetermined minimum pressure differential to actuate the respirator into the inspiratory phase comprises the step of triggering the respirator by the operation of the pressure differential responsive means when the predetermined minimum pressure differential is sensed between the first input and the second input.
 5. A pressure differential triggering system for shifting a respirator from an expiratory phase into an inspiratory phase in response to a patient''s effort to inhale, said system comprising: a patient breathing line providing communication between the patient and the respirator, first and second enclosed chambers, a flexible diaphragm positioned between said first and second chambers and adapted to be flexed in response to a minimum predetermined pressure differential between the pressure in said first chamber and the pressure in said second chamber, means for communicating the pressure in said patient breathing line to said first chamber, modifying means adapted to sense the pressure in said patient breathing line and communicate as a reference pressure to said second chamber, a modified pressure which substantially tracks the patient breathing line presSure communicated to said first chamber, whereby said modified pressure in said second chamber lags the patient breathing line pressure in said first chamber with respect to time and rapid changes in pressure in said patient breathing line cause said minimum predetermined pressure differential between said first and second chambers to flex said diaphragm, and means to sense the flexing of said diaphragm to provide an inspiratory phase triggering signal to the respirator.
 6. The improvement according to claim 5 wherein said modifying means communicates the modified pressure to the second chamber only during a portion of the expiratory phase and comprises a switching valve: said switching valve being operated by a valve actuating means, and said valve actuating means being operable to place said switching valve into a position communicating the modified pressure with the second chamber during a period commencing 0.2 - 0.5 seconds after commencement of the expiratory phase and terminating upon the commencement of the next inspiratory phase.
 7. The improvement according to claim 6 wherein said modifying means includes a pneumatic resistor restricting communicating the pressure sensed in the breathing line with the second chamber.
 8. In a respirator for supplying gas to a patient and having inspiratory and expiratory phases, the respirator being adapted to retain a predetermined positive gas pressure within the patient at the end of the expiratory phase and having switching means to switch the respirator from the expiratory phase to the inspiratory phase, the improvement comprising: a patient breathing line for supplying gas from the respirator to the patient; means to monitor the pressure in the patient breathing line during the expiratory phase, means to sense the pressure in said patient breathing line during a period which commences a predetermined time period after commencement of the expiratory phase; and means to trigger the respirator into the inspiratory phase upon said patient''s attempting to breathe, said means preventing triggering due to pressure declines generated by leaks from said system by effecting said triggering where a drop in said sensed pressure over a sub-interval of said period exceeds a minimum predetermined value; and said means further, disabling said triggering irrespective of the level of said sensed pressure where the average rate of pressure decline over said sub-interval is less than a second predetermined value.
 9. Apparatus for triggering a respirator from an expiratory phase into an inspiratory phase of a breathing cycle in reponse to an effort on the part of a patient to inhale comprising, in combination: a patient breathing line communicating between the respirator and the patient, pressure differential responsive means, means for communicating the pressure in said breathing line to said pressure differential responsive means as a first input thereto, means for modifying the pressure in said patient breathing line and communicating the modified pressure to said pressure differential responsive means only during a portion of an expiratory phase, as a second input to said pressure differential responsive means, said modifying means retarding any pressure variations of the second input with respect to time, whereby the modified pressure comprising the second input lags out of phase with respect to the pressure in the breathing line comprising the first input, and means responsive to a predetermined minimum pressure differential between said first input and said second input in said pressure differential responsive means to trigger the respirator from the expiratory phase into the inspiratory phase.
 10. Apparatus according to claim 9 wherein said pressure differential means comprises a housing, a flexible diaphragm separating said housing into a first chamber for receiving said first input and a second chamber for receiving said second input, And wherein said flexible diaphragm flexes in response to the predetermined rate of pressure drop in said first input to trigger the respirator into an inspiratory phase.
 11. Apparatus according to claim 10 wherein said means for modifying the pressure in said patient breathing line and communicating the modified pressure to said pressure differential responsive means operates only during a portion of an expiratory phase and comprises a switching valve: said switching valve being operated by a valve actuating means, and said valve actuating means being operable to switch said switching valve into a position communicating the second input with said pressure differential responsive means during a period commencing 0.2 - 0.5 seconds after commencement of the expiratory phase and terminating upon the commencement of the next inspiratory phase.
 12. Apparatus according to claim 10 wherein said modifying means includes a pneumatic resistor restricting a gas passageway formed by said means for providing the second input to said pressure differential responsive means.
 13. Apparatus according to claim 11 wherein said modifying means includes a pneumatic capacitor comprising a variable chamber in communication with said second chamber, said variable chamber adapted to be reduced in volume in response to a decrease in said second pressure input. 